A power protection and distribution system is typically installed at every building, factory, or similar facility, where the main electrical power from the grid enters the facility. The power protection and distribution system—also referred to as “switchgear”—usually includes a main circuit breaker at the electrical point furthest upstream, or closest to the external main power grid; a power distribution bus, which may comprise, e.g., copper bars for each positive power phase and one or more ground bus bars; and one or more downstream circuit breakers, each protecting an electrical circuit distributing power to a zone, or area, of the facility. The purpose of the circuit breakers is to protect downstream circuits from overcurrent conditions, such as would occur in the event of a short circuit.
Circuit breakers operate in a variety of ways. For example, one type of circuit breaker determines an overcurrent condition by detecting excessive heat generated by large currents moving through resistive conductors. While such circuit breakers will interrupt a faulty circuit in time to avoid a fire, sensitive downstream electrical equipment may be damaged by the overcurrent condition prior to the circuit breaker interrupting the current flow. Accordingly, to provide a more timely circuit interruption operation, various means of detecting short-circuits and other overcurrent conditions are known in the art. One example is an optical flash detector. In many cases, the cause of a short-circuit condition—such as a foreign object, a rodent, or the like, establishing a conductive path across terminals or nodes of the power distribution bus—will generate a spark, arc, flame, glow of molten metal, or other optical flash. An optical detector, such as a photodetector and associated control circuits, may, upon detection of an optical flash, transmit a signal to an upstream circuit breaker, causing the breaker to trip immediately.
However, optical flashes occur in switchgear that should not necessarily trigger an upstream circuit breaker to trip. For example, air break interruption equipment, such as many circuit breakers, generates an arc as the connection between current-carrying contacts is broken. Thus, a downstream circuit breaker tripping due to an overcurrent condition on the circuit downstream of that circuit breaker may generate an optical flash. In this case, only the affected downstream circuit breaker should trip. However, if an optical flash detector registers an arc or other optical flash when the circuit breaker trips, it will cause the upstream (e.g., main) circuit breaker to trip, unnecessarily shutting down power to the entire facility. This is known as “nuisance tripping,” and should be avoided.
A variety of approaches are known to avoid nuisance tripping. These include controlling placement of optical sensors, such as locating them away from nuisance light sources, or adjusting their placement such that nuisance light sources are out of the field of view of the optical detector; reducing the power system short-circuit current at the switchgear to levels that limit nuisance light production by devices such as circuit breakers; and the like. In practice, these solutions have been found deficient. For example, it may be difficult or impossible to place or orient optical sensors so that they have a full field of view of some optical flashes, while shielding them from others. Also, reducing the current to limit circuit breaker flash intensity also reduces the current generally, which may require duplication of protected circuits to supply the necessary power.
The Background section of this document is provided to place embodiments of the present invention in technological and operational context, to assist those of skill in the art in understanding their scope and utility. Unless explicitly identified as such, no statement herein is admitted to be prior art merely by its inclusion in the Background section.